This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The toluene /o-xylene monooxygenase (ToMO) system catalyzes the NADH dependant mono-oxidation of aromatic compounds like toluene to cresol at a diiron active site. This first catalytic step is essential for the utilization of these hydrocarbons as sources of food and energy for certain bacteria. Extensive studies have been carried to understand how diiron centers carry out their chemistry and assemble all the necessary substrates (electrons, O2, and hydrocarbon) at the active site in the hydroxylase component and control the timing of their reactivity so as to avoid quenching of intermediates. The most recent round of structural studies aimed to identify the pathway in the 250 kDa hydroxylase by which O2 gains access to the diiron center enzyme. Like, ToMO, styrene monooygenase, is essential for initializing the breakdown of aromatics like styrene into useful metabolites. Styrene monooxygenase is a two component flavoprotein. In this system, an NADH- flavin oxidoreductase, SMOB, delivers FADH2 to the oxygenase component, SMOA, where it converts styrene and chemical analogues to epoxides. Recently the structure of SMOA was determined. The current focus is to determine the structure of SMOB and understand how it transfers FADH2 to SMOB.